Stavudine is a potent HIV-1 reverse transcriptase inhibitor licensed in 1994. No systematic evaluation of cerebrospinal fluid stavudine concentrations has been reported, either for single doses or under steady-state conditions. Improved understanding of stavudine penetration into CSF is important because of the desire for adequate inhibitory concentrations in the central nervous system to suppress HIV replication and hopefully to prevent or slow the development of the AIDS dementia complex. This protocol will examine the concentrations of stavudine (d4T) in cerebrospinal fluid (CSF) of HIV-infected subjects undergoing elective lumbar punctures (LP). Two groups of subjects will be studied: 1) those prescribed stavudine for clinical considerations, who will provide insight into CSF concentrations achieved under steady-state conditions; 2) those not receiving stavudine clinically, but agreeing to take a single dose prior to a scheduled LP, who will provide information on acute dynamics. This study will also determine whether or not a relationship exists between patient CD4 cell concentration and efficiency of CSF stavudine entry and whether or not a relationship exists between CSF white cell concentration or total protein and the efficiency of CSF stavudine entry.